A conventional warning device for monitoring a the rpm or speed of a motor internally set up in a system generates an alarm signal to notify the system only when the motor completely stops. Therefore, when the motor is influenced by an external factor or others, there is no alarm signal output when the rotational speed of the motor increases or decreases but does not completely stop. During this period, the motor cannot efficiently dissipate heat so that the system is burned down or will be in trouble due to a heat dissipating problem and no alarm signal having been generated.
FIG. 1 is a block diagram showing a circuit of the conventional warning device for monitoring the rotational speed of the motor which includes a direct current (DC) fan 1, a fan-stopping detector 2, a drive IC (integrated circuit) 3, and an output circuit 4. The fan-stopping detector 2 generates a signal to the drive IC 3 for providing an alarm signal to notify the system having an emergency measure when the motor completely stops. As a warning device like this is influenced by an external factor or others, there is no alarm signal output when the rotational speed increases or decreases but does not completely stop. During this period, the motor cannot efficiently dissipate heat which results in that the system is burned down or put out of order. The present invention can inform the system to take an emergency measure when the rotational speed of the motor increases or decreases because the motor cannot efficiently dissipate heat or operate under a normal condition.